VITAL SIGNS; Risks: Pertussis Protection? Not From the Herd

By NICHOLAS BAKALAR

Published: June 9, 2009

The theory of herd immunity holds that when most people in a group are vaccinated, everyone is protected -- even those who refuse the vaccine, as many families are doing these days out of a belief that vaccinations cause autism and other illnesses. But the theory does not appear to work well with whooping cough.

Researchers studied children enrolled in a Colorado health plan in the period 1996 to 2007, and found 156 laboratory-confirmed cases of pertussis. They recorded the vaccination status of each and matched them to 595 randomly selected control subjects. After controlling for sex, age, season of infection and other factors, they found that the unvaccinated children were about 23 times as likely as vaccinated children to get whooping cough. In other words, about 1 in 20 unvaccinated children were infected, compared with 1 in 500 who were vaccinated. The study appears in the June issue of Pediatrics.

''This study is additional information that both doctors and parents can use,'' said the lead author, Jason M. Glanz, an epidemiologist with Kaiser Permanente. ''Parents need to know the consequences of choosing not to vaccinate.''

Pertussis can cause serious illness, especially in children. According to the Centers for Disease Control and Prevention, more than half of infants who get the disease have to be hospitalized, about 1 in 10 get pneumonia, and about 1 in 50 have convulsions. In 2007, there were 10,454 cases of pertussis reported to the C.D.C., and 10 children died.